This invention generally relates to refractory materials used in pyrometallurgical converters. More particularly, this invention relates to colloidal silica refractory materials used in electric arc furnaces.
Electric arc furnaces use electricity to convert scrap metal, flux, and alloying materials into molten iron and steel. The scrap metal may be any iron or steel material suitable for melting and usually corresponds to the desired composition of the molten metal. The flux materials may be selected to remove impurities, oxidize or deoxidize the molten metal, and perform other refining processes. The alloying materials depend upon the desired composition of the molten metal and the alloy content of the scrap.
A typical electric arc furnace (EAF) has a cylindrical or oval shape with a bowl-shaped bottom or hearth, vertical walls or a shell, and a domed roof made of refractory-lined steel plates. The shell and roof may have water-cooled panels. The shell may have one or more doors and other ports for adding materials, skimming slag, injecting oxygen and fuel such as natural gas and oil, taking tests, and the like. The roof usually opens for charging or loading scrap and other materials onto the hearth. One or more electrodes protrude through holes in the roof into the interior of the shell. Many EAFs have three electrodes arranged in a triangular pattern, forming a xe2x80x9cdelta portionxe2x80x9d in the roof. A mast or other support structure raises and lowers the electrodes above the scrap and molten metal on the hearth. The roof may have an additional opening connected to an exhaust system for removal of off-gases. A tap hole with a refractory-lined spout or trough is on one side of the shell above the hearth. In some EAFs, the tap hole is located elsewhere such as on the bottom of the hearth as in an eccentric bottom tapping (EBT) design.
An EAF is often described by its capabilities such as capacity and transformer size. The capacity of an EAF is often stated as a maximum heat size, which may be up to 200 tons or more. A xe2x80x9cheatxe2x80x9d is one batch or cycle of converting scrap into molten iron or steel. The transformer size correlates to the speed of producing a heat and may be up to 100 MVA or more. EAFs usually produce a heat in the range of about one hour through about three hours. While intermittent operation is possible, most EAF""s are operated continuously with only short maintenance periods.
In operation, the roof opens for charging or loading scrap and other materials onto the electric arc furnace. The roof closes and the electrodes are positioned above the scrap. An electric current is applied through electrodes, forming electric arcs. A single-electrode EAF typically uses direct current and generates electric arcs between the electrode and the hearth. A multiple-electrode EAF typically uses alternating current and generates electric arcs between the electrodes and between the electrodes and the hearth. The arcs generate heat, which causes the scrap and other materials to melt. Additional refining and alloying materials may be added. Oxygen may be injected or blown into the molten metal to remove carbon and other elements. When the composition, temperature, and other specifications are reached, the tap hole is opened and the molten metal is poured or tapped from the EAF into a ladle or a large refractory-lined, bucket-shaped container. If a xe2x80x9chot-heelxe2x80x9d practice is used, a small portion of molten metal is left in the furnace for the next heat or batch of molten metal. After removal from the EAF, the molten metal usually undergoes further processing such as ladle refining, argon-oxygen decarburization, vacuum degassing, and the like.
The refractory material protects the hearth, shell, and roof, and holds scrap and molten metal during the conversion process. The refractory material usually depends on the slag system chosen for the EAF. Slag is a nonmetallic product produced by the reaction of flux materials with nonmetallic impurities. Flux materials generally help melt and separate impurities from the molten metal. The resulting slag floats on the molten metal and may be reactive with the refractory-material. In some slag systems, excess silica may be used and the refractory may be alumina, silica brick, siliceous rock, or the like. In other slag systems, excess magnesia or lime may be used and the refractory may be burnt dolomite, magnesite, or the like. The refractory material may be in the form of bricks, monolithic shapes, castable and pumpable forms, one or more combinations, and the like.
Generally, refractory materials need to withstand the charging of scrap, the tapping of molten metal, melting temperatures of about 3000xc2x0 F., thermal cycling, corrosion and erosion by the molten metal and slag, and other factors associated with pyrometallurgical processes. The number of heats produced before a refractory material is replaced often characterizes the service life of the refractory material. The service life of a refractory also varies depending upon the position of the refractory material in the EAF. Refractory materials with a longer service life are highly desired to reduce the operating costs and increase the productivity of the EAF. The replacement of refractory materials is costly both in materials and labor. More importantly, the replacement of refractory materials idles and thus limits the productivity of an EAF.
Refractory materials in the roof usually have a lower service life because of the operating conditions affecting this part of the EAF. These refractory materials need sufficient structural properties to remain suspended above the hearth and to withstand vibrations from the electric arcs and from opening and closing of the roof. In addition to splashes of metal and slag, these refractory materials also need to be resistant to the off-gases generated during the conversion of the scrap and other materials into molten iron and steel. The off-gases rise above the molten metal, slag, and scrap during the conversion process. Typically, an evacuation system removes off-gases from the furnace. An access hole or port for the evacuation system may be located on the shell or on the roof. The evacuation system essentially pulls the off-gases along the roof refractory prior to exiting the furnace. The off-gases include gases evolved from the conversion process such as carbon monoxide, carbon dioxide, and water vapor. The off-gases also include slag and metal vapors and vapors from the reactions of the flux materials. The off-gases react with the refractory materials in the roof, reducing the structural integrity, thermal resistance, and other properties of the refractory. A portion of the roof adjacent or between the electrodes, the delta portion, essentially bears the worse of these operating conditions. The roof refractory usually comprises one or a combination of alumina-silica and alumina-silica-chromite refractory materials. The delta portion may be made of refractory materials different from the remainder of the roof. The delta portion usually is the first part of the roof to wear out or fail. Refractory materials used in the roof typically have a service life in the range of about 90 through about 120 heats.
This invention provides a colloidal refractory system for an electric arc furnace or another pyrometallurgical converter. The colloidal refractory system has high-temperature strength and resists the attack from slag and off-gases inside the converter. The colloidal refractory system may be used to extend the service life of the roof refractory, especially the delta portion of an electric arc furnace.
A pyrometallurgical converter with the colloidal silica refractory system may have a hearth, a shell, and a roof. The shell is vertically disposed on the hearth. The roof is positioned on the shell and has a refractory portion made of the colloidal silica refractory. The colloidal silica refractory may comprise alumina (Al2O3), silicon carbide (SiC), silica (SiO2), and carbon (C). The Al2O3 has a range of about 55 percent through about 90 percent by weight. The SiC has a range of about 2.5 percent through about 30 percent by weight. The SiO2 has a range of about 2 percent through about 20 percent by weight. The C has a range of about 0.1 percent through about 4 percent by weight.
In a method of manufacturing a colloidal silica refractory, a casting composition is mixed. The casting composition may comprise Al2O3, SiC, C, and an aqueous colloidal silica binder having about 15 percent through about 70 percent of SiO2 by weight. The casting composition is transported into a mold for a refractory portion. The casting composition is cured into a refractory material. The refractory material may comprise Al2O3 in a range of about 55 percent through about 90 percent by weight, SiC in a range of about 2.5 percent through about 30 percent by weight, SiO2 in a range of about 2 percent through about 28 percent by weight, and C in a range of about 0.1 percent through about 4 percent by weight.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.